In  bug 889976  we added a histogram to measure why the simple disk_cache fails to pass its consistency check.  After a week or so we have some initial numbers and they are mostly related to the fake index file.

From most frequent to least:

kBadInitialMagicNumber
kBadFakeIndexFile
kCreateDirectoryFailed
kWriteFakeIndexFileFailed
kReplaceFileFailed

Looking at the fake file index code it seems we may leave a partially written file here:

https://cs.chromium.org/chromium/src/net/disk_cache/simple/simple_version_upgrade.cc?l=145&rcl=c7542cefcc5c4dc18e14e4b10bcfe84396d4819a

That would then trigger a non-recoverable kBadInitialMagicNumber on later opens. (Because that error also means the fake index file is the wrong size...)